The high-pressure treatment of C-60 in an H-2 atmosphere at high temperatures leads to the efficient formation of a covalently bound dimer and some oligomeric species. The resulting hydrogenated C-120 is an example of the bulk production of covalently bound derivatized fullerene cores. Matrix-assisted laser desorption/ionization in conjunction with reflectron time-of-flight mass spectrometry has been applied to the product analysis. The dissociation pattern of selected C120H2x+ ions (x > 30) indicates the dimeric structure of (C60Hx)(2), as opposed to a giant hydrofullerene species possessing a fused C-120 core. However, the results also clearly indicate a much stronger bonding (multiple a bonding) between the C60Hx units than present in cycloaddition products. Evidence of a covalently linked dimer was obtained in labeling experiments, on the basis of which any laser-induced gas-phase aggregation of the C60Hx monomer during the analysis is discounted.